The present invention relates to the general subject of circuits for powering discharge lamps. More particularly, the present invention relates to ballast that efficiently preheats the lamp filaments and that inherently provides lamp fault protection.
Electronic ballasts for gas discharge lamps are often classified into two groups according to how the lamps are ignitedxe2x80x94preheat and instant start. In preheat ballasts, the lamp filaments are preheated at a relatively high level (e.g., 7 volts peak) for a limited period of time (e.g., one second or less) before a moderately high voltage (e.g., 500 volts peak) is applied across the lamp in order to ignite the lamp. In instant start ballasts, the lamp filaments are not preheated, so a higher starting voltage (e.g., 1000 volts peak) is required in order to ignite the lamp. It is generally acknowledged that instant start operation offers certain advantages, such as the ability to ignite the lamp at a lower ambient temperatures and greater energy efficiency (i.e., light output per watt) due to no expenditure of power on filament heating during normal operation of the lamp. On the other hand, instant start operation usually results in considerably lower lamp life than preheat operation.
Because a substantial amount of power is unnecessarily expended on heating the lamp filaments during normal operation of the lamp, it is desirable to have preheat ballasts in which filament power is minimized or eliminated once the lamp has ignited. Currently, there are at least three main approaches for achieving this goal. A first approach, which may be called the xe2x80x9cpassivexe2x80x9d method, heats the filaments via windings on a transformer that also provides the high voltage for igniting the lamp. An acknowledged drawback of this approach is a limit on the degree to which filament heating power may be reduced once the lamp ignites and begins to operate; a detailed discussion of the difficulties with this approach is provided in the xe2x80x9cBackground of the Inventionxe2x80x9d section of U.S. Pat. No. 5,998,930, the relevant portions of which are incorporated herein by reference.
A second approach, which is common in so-called xe2x80x9cprogrammed startxe2x80x9d products, employs an inverter that is operated at one frequency in order to preheat the lamp filaments, then xe2x80x9csweptxe2x80x9d to another frequency in order to ignite and operate the lamp. Because this approach is difficult and/or costly to implement in ballasts having self-oscillating type inverters, it is usually employed only in ballasts having driven type inverters. This approach has the further disadvantage of producing a significant amount of xe2x80x9cglow currentxe2x80x9d through the lamp immediately prior to ignition. Glow current is generally considered to negatively impact the useful life of the lamp.
A third approach employs switching circuitry that disconnects the source of filament power from each of the filaments after the lamp ignites. This approach tends to be rather costly to implement, especially in ballasts that power multiple lamps because multiple switching circuits are required (i.e., one for each filament or each pair of parallel-connected filaments).
All of the aforementioned approaches are largely limited in function to filament heating and do not provide any separate benefits, such as automatic relamping capability or prevention of the high voltages, currents, and power dissipation that generally occurs following lamp removal or failure. Because ballasts that implement these approaches generally require separate, dedicated circuitry in order to accommodate relamping and protect the ballast from damage due to lamp removal or failure, the resulting ballasts tend to be functionally and structurally complex.
What is needed, therefore, is a ballast in which: (i) the filaments are properly preheated prior to lamp ignition; (ii) little or no power is expended on filament heating during normal operation of the lamp; and (iii) little or no pre-ignition glow current occurs. A need also exists for a filament heating reduction approach that is readily implemented in ballasts having either driven or self-oscillating inverters. A further need exists for a filament heating reduction approach that accommodates relamping and that provides lamp fault protection without requiring extensive additional circuitry. A ballast with these attributes would represent a significant advance over the prior art.